This invention relates primarily to threaded fasteners having heads with wrenching surfaces. In particular, the invention relates to a fastener which has an annular protrusion disposed adjacent to the wrenching surface, the protrusion providing a means for stabilizing the fastener during driving. The invention has particular application for screws having self-drilling tips.
Typically, fasteners having external wrenching surfaces, such as hex-head fasteners, are manufactured with a dimension between oppositely facing surfaces which is substantially less than the corresponding dimension of a driving socket. Obviously, the difference between these dimensions is intended to allow insertion of the fastener head into the socket. This difference in dimension or play creates a situation which allows pivoting of the socket around the fastener head. In a drilling screw, this creates instability if there is any eccentricity in the drilling point relative to the fastener head. Fasteners having hexagonal wrenching surfaces often include an integral washer which is sloped in the direction of the drilling in order to create a recess on the underside of the integral washer. Drivers used to drive such fasteners frequently have flat end surfaces. This combination of shapes means that a line of contact between the sloping washer and the flat end surface, occurs in the inner edge of the flat end surface of the driver. As the surface of the washer slopes away from the flat end surface, a clearance is created which allows tilting of the driver.
The fastener of the present invention addresses this problem by forming a ring disposed on the sloping surface of the integral washer. The ring engages the central portion of the flat end surface of the driver, and substantially reduces the tendency of the driver to tilt about the head of the fastener. In a preferred embodiment, the ring is stamped into a stainless steel head cap which is placed over a carbon steel fastener head.
The advantages of the present invention will be better understood upon a reading of the following specifications read in conjunction with the attached drawing, wherein: